Vertical Microbial Transport via Water Flow Through Beach Sediments

M. Phillips, H. M. Solo-Gabriele, A. Reniers, T. Kiger, A. Shah

Center for Oceans and Human Health Rosenstiel School of Atmospheric and Marine Science, University of Miami

Methods Results Introduction

   Enterococci are used by the EPA to test recreational waters for human fecal contamination Can also grow in beach sediments and contaminate water without point source of sewage Process by which microbes are transported from the sediment into the water column has not been widely studied

Objectives

  Determine the contribution of vertical entrainment in transporting enterococci into the water column Determine if enterococci are transported into the water column in a predictable manner

Site Description

    Hobie Beach, Virginia Key, Florida No point source of sewage Closed 4.6 days per year due to microbial water quality violations Epidemiological studies have shown illness correlated to bathing at Hobie Beach 

One large experimental core and three small cores were sampled per sampling event

Flow-Through Columns

Up-Flow Column Down-Flow Column Experimental

Sampling Experiments and Analysis

 Sterile water was driven through experimental cores via pressure created by head differential  Water samples were collected from eluent that had passed through the core  Water samples were then analyzed for enterococci via Membrane Filtration (EPA Method 1600)  Microbes from sand (before and after) also quantified  Moisture Content, Volatile Organic Content, Grain Size Analysis, Porosity and Flow Rate also measured  Hydraulic conductivity (measure of difficulty for flow to pass through sediment) was calculated using Darcy’s Law

Dislodge-able Bacteria Released Quickly

4,0% 3,5% 3,0% 2,5% 2,0% 1,5% 1,0% 0,5% 0,0% 0 100 200 300

Volume Eluted (mL)

400 500 600 2,5%

Release Pattern Did Not Differ With Direction of Flow

7/21/2009 8/25/2009 8/28/2009 11/2/2009 11/9/2009 11/23/2009 12/8/2009 12/17/2009 1/12/2010 1/23/2010 2,0% 1,5% 1,0% 0,5% Up Flow Mean (Dashed Stdv) Down Flow Mean (Solid Stdv) 0,4 0,35 0,3 0,25 0,2 0,15 0,1 0,05 0 0 0,0% 0 100 200 300

Volume Eluted (mL)

400 500

Hydraulic Conductivity Correlated with Percent Bacteria Removed

y = 0,011x - 0,0306 R² = 0,5065 5 10 15 20

Percentage of Total Bacteria Removed

25

Explosive Growth After Rain Event

 One experimental Core was collected during heavy rain  Quantity of enterococci and percentage released significantly higher than other cores

Rain Core vs Means of Other Cores

CFU/g Dry Sand Before CFUs Washed off Core % Removed from Core *CFU- Colony Forming Unit, the functional unit for measuring bacteria Average 4 659 11% Rain Core 458 204600 40% 600 30

Summary

 Only 10% (on average) of total enterococci in experimental core were dislodged and washed off  98% of dislodge-able enterococci released within one pore water volume (400mL)  Direction of flow and flow velocity have no effect on percentage of enterococci released  Mild correlation seen between hydraulic conductivity and percentage of enterococci released  No correlation seen with any other physical or chemical parameters

Conclusions

 Results suggest some other parameter controls percentage of enterococci released  Biofilms could be this parameter  Rain events fundamentally alter the amount of bacteria quantified from sand  Run off and/or different states of cultivability in enterococci could explain difference

Recommendations

 Future work should quantify biofilms along with bacteria to establish correlation  Microscopy can be used to determine if enterococci in different culturable states are present in sand samples  Since vertical entrainment produces no predictable release of bacteria, other physical forces such as shear should be examined for possible use in a predictive model

Acknowledgements

This work was funded by NSF (#OCE0432368/0911373) and NIEHS (1 P50 ES12736)